1. Field of the Invention
The present invention relates to a flowmeter for measuring the rate of flow of a fluid through a passage on the basis of vibrations of the fluid as it flows through the passage.
2. Description of the Prior Art
Various flowmeters are known for indirectly measuring the rate of flow of a fluid by determining its speed of flow by way of measurement of the frequency of vibrations of the fluid which are caused within a passage of the meter, the vibrations being substantially proportional to the speed of flow of the fluid through the passage. Flowmeters of this kind include Karman vortex flowmeter, fluidic flowmeters, and rotational air flow meters.
These flowmeters are subjected to less measuring errors than other types of flowmeter such as orifice flowmeters and hot-wire flowmeters. More specifically, the orifice flowmeter includes a pressure sensor for detecting the pressure of a fluid at a throat. Since the output of the pressure sensor is a quadratic function of the rate of flow of the fluid, the orifice flowmeter suffers from large detecting errors in the range of low rates of fluid flow. The hot-wire flowmeter utilizes a hot wire for direct measurement of the speed of flow of a fluid. Attempts to improve responsiveness of the flow meter require that the hot wire sensor be thin and small in size. Such arrangement however results in poor durability. The hot-wire flowmeter is also disadvantageous in that it needs a linearization device for electric arithmetic operations which lead to a complex computation and display processing.
Flowmeters which operate on the basis of fluid vibations are only required to detect the period of such vibrations for measuring either pressures or speeds of flow. No high precision for the output of a sensor is necessary for flowmeters in this category, and since the measured period of vibrations is in linear relation to the speed or the rate of flow of the fluid, arithmetic operations required to derive the rate of flow are quite simple and flowmeter apparatus are inexpensive to construct.
The Karman vortex flowmeter includes a blunt-headed body disposed in a flow passage for producing Karman vortices behind the blunt-headed body, and the frequency of generations of Karman vortices is measured to find the rate of flow. For stable formation of Karman vortices, the shape and structure of the blunt-headed body are designed or selected to suit the fluid passage and the fluid to be measured. In some applications, the blunt-headed body has a fluid passage therethrough, or the positional relationship between the blunt-headed body and the wall of the flow passage is optimized. Although the above structural designs are easily comprehensible from the theoretical standpoint, various difficulties are encountered to provide actual arrangements to form Karman vortices stably.
The fluidic flowmeter is characterized in that symmetrical feedback loop openings are defined in sidewalls of a flow passage for a main fluid to inject control fluid flows from the feedback loops into the main fluid for periodically attaching the latter alternately to the sidewalls. The rate of flow of the main fluid can be determined by detecting the period of such periodic oscillation of the main fluid. Although fluid vibrations are relatively stable, control fluid flows injected from the feedback loops into the main fluid are required to have a sufficient controlling effect on the main flow in order to oscillate the main fluid flow stably into alternate attachment to the sidewalls. The fluidic flowmeter is therefore suitable for use with main fluid passages having a relatively small diameter, but is not applicable to fluid passages of a relatively large diameter handling a large amount of fluid.
The rotational air flow meter has a guide in an inlet of the meter for generating a swirling fluid flow within a venturi. The guide however also serves to give resistance to the flow of a fluid, and results in a complicated structure.